It is known that it is often desirable to treat plaster in bulk before use or to treat a plaster-based product such as plaster-fibreboard before manufacture, in order to protect the finished material against water penetration resulting, for example, from water seeping up from the ground. A bulk treatment of this type takes place by adding a waterproofing composition to the mixing or guaging water when the plaster is manufactured.
Potassium or sodium siliconates may be mentioned among waterproofing substances which have already been used. As a result of their alkalinity, these products have the disadvantage that they produce surface effloresence, or even pits, owing to the presence of soluble carbonates. Attempts have also been made to use powdered silicone resins but they are difficult to use owing to the fact that the difference in density of the powders may range from 1 to 3 and also to the fact that powdered cross-linking agents or catalysts have to be used. Different types of aqueous emulsions of silicones have also been used; but in this case the rate of waterproofing for a reasonable consumption of emulsion is inadequate and, in addition, the amount of water absorbed remains relatively large (of the order between 5 and 7%, whereas it varies between 15 and 20% for non-waterproofed plaster).
It has also very recently been considered to use silanes in appropriate forms which are incorporated in the mixing of the plaster. After hydrolysis the silanes lead to the formation in situ of a silicone-type of resin. At present there are several impediments preventing the generalized use of silanes as a waterproofing product. In practice the silanes are in the form of a volatile and toxic liquid which is effectively not soluble or dispersible in water; they cannot therefore be used separately but only in conjunction with other constitutents which permit their use. In addition silanes are relatively expensive and thus they should be used in the optimum manner, avoiding as far as possible their loss by evaporation and facilitating their contact with the mixing water.
In French Patent Application No. 80 26515 filed on Dec. 12, 1980, it is proposed to use a silane with a tertiary solvent for rendering the silane soluble in water. Owing to the tertiary solvent, good contact results between the mixing water and the small amount of silane added thereto, whereby efficient waterproofing of the plaster or the plaster-based product is obtained. Nevertheless, in the finished material, the tertiary solvent, which is an alcohol with a low molecular weight, such as methanol or ethanol, or a ketone, such as acetone, tends to evaporate and entrain the silane, which results in a non-negligible loss of silane, taking into account its high cost.
In order to facilitate hydrolysis of the silane in an aqueous medium and to ensure at the same time a good dispersion of the silicone resin formed inside the plaster or plaster-based product, it is also proposed in the above-mentioned French application to add to the mixing water a slightly acid acidification agent, such as acidic calcium mono- or di-phosphate. Although this acid agent does, however, assist the hydrolysis of the silane it has the disadvantage that it retards the setting of the plaster, whereas in many cases it is advantageous to accelerate this setting process.